1. Field of the Invention
The present invention relates to a low pressure discharge lamp apparatus and in particular, a lamp apparatus effective as an ultraviolet light source for produced by a photochemical reaction.
2. Description of the Related Art
The technique for producing a photochemical reaction emitting of ultraviolet radiation found its application in the various fields of the art. In the semiconductor manufacturing technique, various methods have been extensively used, such as a photochemical vapor deposition method for use in the growth of a thin silicon film on a semiconductor substrate, ultraviolet radiation for use in the polymerization and curing of a photo-resist, photoashing using ultraviolet radiation, and washing using light.
Even in those fields, such as the purification of water, sterilization treatment, and pasteurization treatment of meat and flesh, the technique for performing various treatments using ultraviolet radiation of a short wavelength for illumination has been rapidly developed.
In these respective fields, there is a growing demand for those light sources for emitting ultraviolet radiation of a short wavelength for effective illumination. A low pressure mercury discharge lamp is employed for that purpose.
The low pressure mercury discharge lamp has a discharge tube of quartz glass transmissive to ultraviolet radiation and a pair of electrodesone at each end. A mercury/rare gas is sealed in the discharge tube and, when discharge occurs at the mercury vapor in low pressure, ultraviolet radiation of a short wavelength of 185 nm and 254 nm, a resonant line of the mercury, is emitted.
In recent times, improved treating capability and treating speed have been demanded in the field using the aforementioned photochemical reaction technique. To meet these requirements, it is necessary to increase the intensity of ultraviolet radiation emitted from the lamp.
A high power output or a very high power output lamp is desired through an increase in a light output per unit length or per unit area of the lamp.
The discharge lamp has, in general, a property that an increase in the input of the lamp results in an increase in the light output. With I (ampere) and S (cm.sup.2) representing current supplied to the lamp and a cross-sectional area of the discharge space, respectively, an increase in the current density leads to an increase in an amount of emission of ultraviolet radiation.
In the conventional lamp, however, an increase in the density of current causes an increase in the output of the ultraviolet radiation for some level range and no increase in the output of the ultraviolet radiation, for a range greater than a given level. There is also a tendency that an increase in the lamp input leads to a drop in the output of the ultraviolet radiation, a phenomenon where the amount of emission of the ultraviolet radiation is saturated, even though the mercury vapour pressure is controlled to stay constant.
The reason for this is assumed to be as follows:
The conventional low pressure mercury discharge lamp has been employed such that it is connected to a lighting circuit apparatus using a reactor element such as a choke coil's ballast. The lighting circuit apparatus using a reactor element employs a commercial power supply voltage in a boosted or reduced level. Therefore, a sinusoidal waveform current is supplied from the lighting circuit to the lamp.
The sinusoidal waveform current has a low current area in which it never reaches an effective current value (root mean never square value), effective to ionize when positive and negative polarities are reversed. As a result, the density of the ionized mercury is lowered, and mercury atoms are less excited. Hence, the excited mercury density is lowered.
The light emission of the mercury occurs due to the impact of electrons against mercury atoms. The lowering of the aforementioned electron density corresponded to the decrease of excited mercury atoms and of the excited mercury density. This lowers the light output and hinders the output of the ultraviolet radiation.
Even if a lamp supply current is increased above a given level, a lowering in the mercury density and a drop in the ratio of the electron temperature are rather great, a state that the output of the ultraviolet radiation is saturated.
For the electric current of the sinusoidal waveform, a peak current flows in the lamps discharge path at a level about .sqroot.2 times the effective value (root means square value) effective to the ionization. The .sqroot.2 times peak current excessively ionizes the mercury atoms in the arc and the amount of excited mercury atoms is removed in the ionization, lowering an amount of excited mercury atoms. The emission of the ultraviolet radiation occurs when the excited mercury atoms return back to a steady-state. As the amount of excited mercury conducive to the aforementioned light emission becomes smaller, the amount of emission of the ultraviolet radiation is lowered. Further, an increase in the peak current leads to a drop in the electron temperature, making the excitation of mercury atoms more and more difficult.
In order to replenish the amount of excited mercury atoms it is necessary to accelerate the evaporation of mercury and to heighten the mercury vapor pressure. However, the mercury vapor pressure has an optimum range in which the luminous efficiency becomes maximal. Therefore, when out of this optimum range, the mercury vapor pressure becomes too high, the emitted ultraviolet radiation is absorbed in a high-density mercury vapor in the arc, and the ultraviolet radiation is lowered due to the "self-absorption".
Since, in the conventional case, a sinusoidal current flows in the lamp, even if the current density of the lamp is improved, it is not possible it is reached to, once it is reached to a maximum level is reached, to heighten the intensity of the ultraviolet radiation beyond the maximum level. Thus, the maximum output level of the ultraviolet radiation is low.